In this long-term investigation we are utilizing the naturally occurring mirror display of one variety of squirrel monkey as a means of identifying genetically constituted forms of behavior used in social communication. In previous work on this project it was shown that the integrated performance of the display depends on neural systems that converge in the medial pallidal segment of the striatal complex, a phylogenetically ancient formation of the forebrain derived from reptiles. The medial pallidal segment projects to the dorsal thalamus and to parts of the thalamic and midbrain tegmentum. The present experiments represent a continuing effort to identify core gray matter and tegmental structures in the caudal thalamus, midbrain, and isthmus region involved in the three main aspects of the display--namely, vocalization, thigh-spread, and genital tumescence. The findings indicate that the core gray matter and the immediately adjacent tegmentum in the caudal thalamus, as well as in the caudal part of the midbrain adjoining the isthmus region, is particularly implicated in the vocal aspects of the display. In regard to special nuclei associated with the isthmus region, it has been shown that the serotonin-containing cells of the superior central nucleus of Bechterew, as well as the ascending system from the dorsal tegmental nucleus of Gudden, are not essential for the complete and consistent performance of the display.